Sodium chlorite is a strong oxidizer which is manufactured as either a solid or as an aqueous solution. It is used in various well-known processes, for example, to bleach textiles, and to oxidize contaminants in drinking water and waste water. As described in chapter 12 of the Fourth Edition of Handbook of Chlorination and Alternative Disinfectants edited by George Clifford White sodium chlorite is also used to produce chlorine dioxide which is a powerful and selective oxidant and disinfectant. Chlorine dioxide produced in these processes is used for many purposes including, treatment of drinking water and waste water, disinfection and preservation of food, and sterilization of medical devices as disclosed in U.S. Pat. No. 4,681,739. In many of these processes, sodium chlorite is supplied as an aqueous solution. In some processes, such as the Gas:Solid process described in the White reference, sodium chlorite is used as a granular solid. Sodium chlorite is also supplied as a solid to some users who dissolve it near the point of use and use it as an aqueous solution.
U.S. Pat. No. 5,110,580 teaches that in order for the Gas:Solid process to function efficiently two conditions must be satisfied. First, the gas must be humidified. Otherwise, the patentees teach, the Gas:Solid reaction may cease to function. Secondly, pre-treatment of the sodium chlorite is necessary to remove some or all of the sodium hydroxide. Otherwise, the chlorine flowing through the solid bed will preferentially react with the sodium hydroxide until the hydroxide is consumed and start-up of the process will be delayed.
U.S. Pat. No. 4,044,103 describes a process for making storage-stable sodium chlorite. In this process a salt which forms a hydrate is added to sodium chlorite with water and then dried. The final composition of the material is such that it contains a weight of water (as hydrate) equal to at least about 5% of the anhydrous weight of the sodium chlorite. If this criteria is met, patentees teach that the resulting sodium chlorite formulation does not propagate decomposition upon sudden local heating, even to a high temperature. Patentees describe various ways to mix the sodium chlorite and the hydrating salt including, adding dry ground hydrated salt to a paste of sodium chlorite and drying the resulting paste, and adding a solution of hydrating salt to dry sodium chlorite and drying the resulting paste.
It is important to note that all of the processes taught in the ""103 patent use large amounts of excess water which is evaporated away at elevated temperatures (typically 45xc2x0 C.) after the salts are mixed. Patentees also teach the use of various hydrating salts with high melting points and combinations of salts with high melting points to prevent melting of the product in hot weather. In all of the examples cited, drying of the product is required, typically at 45xc2x0 C.
The present invention is a process for manufacturing agglomerated sodium chlorite particles suitable for, inter alia, producing chlorine dioxide by the Gas:Solid process. A particular useful form of agglomerated particles are pellets. The pellets manufactured according to the present invention do not propagate the decomposition reaction of sodium chlorite even when locally heated to very high temperatures. These pellets resist melting at temperatures normally found in the work environment. Compared to other sodium chlorite formulations these pellets are more resistant to combustion when mixed with a fuel. Also the pellets are approximately uniform in size and are approximately spherical in shape. The pellets produced by the present invention permit uniform flow of gas through a bed of the pellets with less pressure drop than a comparable bed of flakes of the same composition. The pellets produced according to the present invention are permeable to the mass transfer of gas into and out of the particles during the Gas:Solid process. They dissolve rapidly when mixed with water, do not require humidification of the gas when used in the Gas:Solid process, and do not require pretreatment for removal of hydroxide in order to achieve rapid start-up of chlorine dioxide production in the Gas:Solid process.
Thus, in one aspect the present invention is a method for producing a stable form of sodium chlorite comprising the steps of, introducing water and granular sodium chlorite combined with one of a metal salt or salts that form hydrates in an amount sufficient so that when fully hydrated the water of hydration is more than about 5% of the anhydrous weight of the sodium chlorite with water the amount of water being less than the weight of water required to fully hydrate the salts, but more than 5% of the anhydrous weight of the sodium chlorite, into a pelletizing apparatus and recovering pellets being of a size to pass a three mesh screen pellet.
The present invention includes using a salt that is also sufficiently soluble in water having a melting point above 50xc2x0 C. added in solution form in an amount to raise the melting point of the final particles to more than 50xc2x0 C., while adding less water than required to fully hydrate the full amount of hydrating salts in the mixture.
In another aspect the present invention is a method of producing chlorine dioxide gas by passing a dilute mixture of chlorine gas with an inert gas being one of air or nitrogen through a bed of sodium chlorite pellets containing water of hydration greater than about 5% of the anhydrous weight of sodium chlorite.